We started to write our own implementation of mikktspace, but it
required an unindexed mesh for input. This was awkward because assimp is
already applying a pipeline of operations that includes tangent
generation and indexing. It felt reduandant to add our own pipeline on
top of that (unindexing => tangentgen => reindexing).
Since assimp's CalcTangentsProcess method is already producing
tangents that follow the orientation implied by texture coordinates,
this simply needs to be tweaked to match glTF. (flip the bitangent)
Also fixed a bug for the case where models have neither UV's nor
tangents, where we were calling norm instead of normalize. :)
0b84425bd0